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Creating Fractals Using Iterated Function Systems Construction of Fractal Objects with Iterated Function Systems Nicolas Dutil

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Creating Fractals Using Iterated Function Systems Construction of Fractal Objects with Iterated Function Systems Nicolas Dutil Creating Fractals using Iterated Function Systems Construction of Fractal Objects with Iterated Function Systems Nicolas Dutil 1-Introduction: Suppose you had no ideas what fractals were, and you were asked to model objects like clouds, grass, plants, etc.. What else could you use to define the geometries of these objects? Although polynomials can easily define objects with smooth geometry, they are pretty useless if you want to model complex objects like grass or plants, since those possess infinitely non smooth, highly structured geometries. In the early 1980s, mathematicians have become concerned with non-smooth sets, that is, sets where the method of classical calculus couldn't be applied. It was the fundamental work of Mandelbrot that opened up a new way to model natural phenomena. What is a fractal? Many definitions exists, and mathematicians have not yet agreed on one. Benoit Mandelbrot refers to the word "fractals" as objects who possess self-similarity. For the rest of this paper, we will adopt this as our definition of a fractal. It is important for the reader to understand that fractals don't really exists in nature. They are used as a model for studying complex natural phenomena. If you look closer and closer at a leaf of a tree, you will eventually arrive at the cells that constitute the leaf. This property of self-similarity that natural objects possess is maintained as long as we don't look to close at them. However, with fractals, there is no limit as too how far we can look before this property disappears. As we keep looking deeper and deeper in the structure, additional details will be revealed and you will probably have a strong feeling of déjà vu. This brings us to our main discussion: how can we generate these fractals? Besides using IFS, which is our main topic, other techniques exists. Lindenmayer systems, or L-systems for short, where invented in 1968 by Aristid Lindenmayer [Lind68] as a way to model biological growth. The technique works as follows : we have an alphabet V consisting of various symbols and an initiator (a string of symbols from V) to which we apply a list of production rules. By applying all the rules, we create a new word. If we repeat this scheme over and over again, a global pattern will emerge. This global process will often have the self-similarity property that we were talking about. This technique is very similar to an iterated function system. Both of them are dynamical systems (defined later). An Iterated Function Systems is a set of contraction mappings W={w1,w2,...wn} acting on a space X. Associated with this set of mappings W, is a set of probabilities P={P1,P2,...,Pn}. As we will see, these probabilities are used to generate a random walk in the space X. If we start with any point in X and apply these maps iteratively, we will come arbitrarily close to a set of points A in X called the attractor of the IFS. These attractors are very often fractal. (for the most part, we will assume attractors are fractal sets, and thus, use the words interchangeably) This forms the basis for creating an algorithm that will approximate the attractor of an IFS. We sometime call sets {Wk(A)} whose limits are fractals, pre-fractals. These are sets the algorithm will be able to generate . Increasing the number of times we apply the maps will give us a more accurate picture of what the attractor looks like. file:///C|/project.html (1 of 10) [10/04/2000 19:11:38] Creating Fractals using Iterated Function Systems 2-Mathematical Preliminaries In order to understand what iterated function systems are and why the random iteration algorithm works, we need to be familiar with some mathematical concepts. Readers with a good background in analysis and algebra can move on to the next section. For the rest of this section, a space X is simply a set of elements (points). I- Metric Spaces definition: A space X with a real-valued function d: X x X →ℜ is called a metric space (X,d) if d possess the following properties: 1.d(x,y) ≥ 0 for ∀ x,y ∈ X 2.d(x,y) = d(y,x) ∀ x,y ∈ X 3.d(x,y) ≤ d(x,z) + d(z,y)∀ x,y,z ∈ X . (triangle inequality) For instance, ℜ with d = |x-y| is a metric space. ℜ2 with the usual euclidian distance is also a metric space. Open Sets definition : A subset S of the metric space (X,d) is open if, for each point x ∈ S , we can find a r > 0 so that {y ∈ X : d(x,y) < r } is contained in S. Closed Sets definition:A subset S of the metric space (X,d) is closed if,whenever a sequence{xn}contained in S converges to a limit x ∈X,then in fact this limit x∈S. Bounded Sets definition: A subset S of the metric space (X,d) is bounded if we can find an x ∈ X and an M ∈ℜ > 0 so that d(a,x) ≤ M ∀ a ∈ S. Cauchy Sequence ε ∈Ν definition: A sequence {xn} in X is called a Cauchy sequence if given > 0, we can find an N > 0 <ε ∀ > Ν such that d(xn,xm) n,m Note: A cauchy sequence need not have a limit in X. This stimulate the next definition. Complete Metric Space definition: A metric space (X,d) is complete if every Cauchy sequences in X converges in X. Compact Sets definition: A subset S of the metric space (X,d) is compact if every sequences in S has a subsequence which converges in S. Since we are mostly concerned with metric spaces where the underlying space is ℜn or Cn , we can state the following : theorem: If a subset S ⊂ ℜn (or Cn) is closed and bounded, then it is compact. Now that we know what a compact set is, we can define the following space: file:///C|/project.html (2 of 10) [10/04/2000 19:11:38] Creating Fractals using Iterated Function Systems definition: Let X be a complete metric space.Then H(X) consists of the non-empty compact subsets of X. To make H(X) into a metric space, we must find a real valued function h : H(X) x H(X) → ℜ with the properties enumerated before. To construct this metric, we need to know what a δ-parallel body Aδ of a set A is : definition : Let (X,d) be a metric space. A δ-parallel body Aδ of a set A is the set of points in X within distance d of A : {x ∈ X : d(x,y) ≤ d for some y ∈ A} definition : Let A,B ∈ H(X), and h(A,B) = infinimum{ d : A ⊂ Bδ and B ⊂ Aδ } . We call h(A,B) the Hausdorff metric. The Hausdorff metric tells us how close two sets are to each other. If A and B are very close, then d will be small, and thus h(A,B) will be small also. Before we can begin talking about iterated function systems,there's two more interesting mathematical notions we must be aware of. II- Dynamical Systems definition : A dynamical system is a transformation S : X → X on a metric space (X,d). It is denoted by {X ;S}. definition : Let {X ;S} be a dynamical systems. A point x ∈ X for which S(x) = x is called a fixed point of {X ;S}. III- Contraction Mappings definition : Let S : X → X be a transformation on the metric space (X,d). S is a contraction if ∃ s ∈ ℜwith 0 ≤ s < 1 such that d(S(x),S(y)) ≤ sd(x,y) ∀ x,y ∈ X. Any such number s is called a contractivity factor of S. The following theorem will be very important for later on. Contraction Theorem : Let S : X → X be a contraction on a complete metric space (X,d). Then S ∈ ∈ k possess exactly one fixed point xs X and moreover for any point x X, the sequence {S (x): n = → k 0,1,2.....} converges to xs. That is, limn inf S (x) = xs . The proof can be found in [Barn93] 3-Iterated Function Systems [Barn93] defines an iterated function systems in the following way : definition : A (hyperbolic) iterated function system consists of a complete metric space (X,d) together → with a finite set of contraction mappings wn : X X, with respective contractivity factor sn , for n = 1,2,....N. The abbreviation "IFS" is used for "iterated function systems". The notation for the IFS just announced is {X ;wn : n = 1,2,...N} and its contractivity factor is s = max{sn :n = 1,2,...N}. The following theorem is extremely important and suggest an algorithm for computing the pre-fractals. file:///C|/project.html (3 of 10) [10/04/2000 19:11:38] Creating Fractals using Iterated Function Systems Theorem 3.1: Let {H(X) ;w1,w2,...wN}be an IFS with contractivity factor s. We define W(B) = union(wi(B)) for i=1..N, B∈ H(X). Then the following can be said : a) W(B) is a contraction mapping with contractivity factor s b) Its unique fixed point A ∈ H(X) obeys A = W(A) = union(wi(A)) for i = 1..N k k ∈ given by A = limk→infinityW (B) = intersection(W (B)) k=1..infinity, for any B H(X). The proof can be found in [Barn93]. definition : The fixed point A ∈ H(X) as described in the theorem is called the attractor of the IFS.
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